1. Field of Invention
The invention relates to rotary drill bits for use in drilling holes in subsurface formations, and of the kind comprising a bit body having a leading face and a gauge region, a plurality of blades formed on the leading face of the bit and extending outwardly away from the axis of the bit towards the gauge region so as to define between the blades a plurality of fluid channels leading towards the gauge region, a plurality of cutting elements mounted along each blade, and a plurality of nozzles in the bit body for supplying drilling fluid to the leading face of the bit for cleaning and cooling the cutting elements.
2. Description of Related Art
The invention is particularly, but not exclusively, applicable to drill bits in which some or all of the cutters are preform (PDC) cutters each formed, at least in part, from polycrystalline diamond. One common form of cutter comprises a tablet, usually circular or part-circular, made up of a superhard table of polycrystalline diamond, providing the front cutting face of the element, bonded to a substrate which is usually of cemented tungsten carbide.
The bit body may be machined from solid metal, usually steel, or may be moulded using a powder metallurgy process in which tungsten carbide powder is infiltrated with a metal alloy binder in a furnace so as to form a hard matrix.
Hitherto in drill bits of this kind it has been usual for the leading edge of each blade, along which the cutting elements are mounted, to be substantially straight so that the cutting elements also extend in a substantially straight line, as viewed axially of the drill bit, as they extend outwardly away from the central axis of the bit. Often the leading edges of the blades extend generally radially, although arrangements are known in which the leading edges of the blades are inclined forwardly or rearwardly of the radius which passes through the inner end of the leading edge.
The present invention sets out to provide a novel arrangement whereby the leading edges of at least some of the blades on the drill bit are curved, which may provide certain advantages as will be explained hereinafter.